It is in principle known to treat room air or breathing air with what are known as ionizers, in order to reduce pollutants. Pollutants and odorous substances usually form complex and large molecules, which are broken down by the ionizer into small molecular fragments. At the same time, radicals, in particular oxygen radicals, form as a result of the ionization, and these radicals can then oxidize with the broken-down fragments. The ionizer operates on the basis of a controlled gas discharge that takes places between two electrodes and a dielectric located therebetween. The gas discharge is a barrier discharge, the dielectric acting as a dielectric barrier. Individual discharges that are limited with respect to time and are preferably distributed homogeneously over the entire electrode surface are thus attained. It is characteristic of these barrier discharges that the transition into a thermal arc discharge is prevented by the dielectric barrier. The discharge is interrupted before the high-energy electrons (1-10 eV) that arise during the striking process release their energy to the surrounding gas as a result of thermalization.
In the household sector, in particular, various applications for an air purification device of this type have been proposed in the past. For example, it is known from DE 198 10 497 A1 to provide an air purification device of this type in a toilet, in order to eliminate odors. For this purpose, suitable suction devices comprising air ducts on the upper flushing rim of the WC-bowl or in a hollow channel in the toilet seat lead the contaminated air to the ionizer, in order to reduce the odor contamination.
One problem in the operation of the ionizer is that of activating the ionizer at an ionization power suitable to requirements. If the ionizer is acted on by too little ionization power, the ionization is unsatisfactorily low, whereas if ionization is too high, too many ions and radicals, which leave the operator with the impression of the odor of a pungent corrosive or cleansing agent, are sometimes released. In this operating state, in addition to the formation of ions, there is also the production of ozone, the excessive production of which is likewise undesirable.
In order to solve this problem, WO 98/26482 describes an air purification device comprising an ionizer, the supply voltage of which is controlled via a gas sensor. The gas sensor is a metal oxide semiconductor sensor, the resistance of which decreases as the concentration of specific gases (generally oxidizable gases or vapors, such as hydrogen sulphide, hydrogen, ammonia, ethanol or carbon monoxide, for example) increases. The variation in resistance is thus a measure of the contamination of the air with specific pollutants. According to WO 98/26482, as the pollutant concentration rises, the ionization power by which the ionizer is acted on is increased in a sensor-controlled manner up to a maximum value. In other words, at a low pollutant concentration measured by the gas sensor, the ionizer is acted on by a correspondingly low ionization power, whereas at a high pollutant concentration measured by the gas sensor, the ionizer is also activated at a correspondingly high ionization power. In order to supplement this sensor control, WO 98/26482 also describes the use of an additional ionization sensor and/or ozone sensor. Since it is a prerequisite that the air quality sensor in the sensor control measures the pollutant concentration of the supplied air and is thus arranged, in terms of the flow technology, upstream of the ionizer, the purpose of the additional ionization sensor and/or ozone sensor is to identify an ozone concentration, which is still undesirable, in the purified air, in order then optionally to correct the ionization power as appropriate.
A sensor control corresponding to WO 98/26482 is also described in DE 43 34 956 A1. DE 43 34 956 A1 proposes a tin oxide gas sensor that detects the oxidizable room air components. If this gas sensor identifies a relatively high degree of room contamination, then the ionizer is also activated at a relatively high ionization power. The use of a moisture sensor and a flow sensor is also proposed, in order to increase the ionization power even if a relatively large volume of air or a relatively high air moisture content is measured.
One drawback of the control methods known from WO 98/26482 and DE 43 34 956 A1 is the fact that the gas sensors used have a limited measuring range and also a comparatively slow reaction time. As a result of the limited measuring range, sensor control of the ionization power is not possible in the peripheral regions of the measuring range. If, for example, the pollutant concentration is below the lowest measurement value of the gas sensor, the ionizer is either switched off or the ionization power continues to be operated at a predetermined minimum value. In the event of rapidly varying pollutant concentrations, the slow reaction time of the sensor also means that the ionizer is only activated according to requirements after a certain delay. In the elimination of odors in a toilet, for example, this delay is disadvantageous, since precisely in the event of a precipitous increase in odorous substances, immediate elimination of the odorous substances by means of the ionizer is desirable.